The major objective of the proposed research is to understand the molecular biology of regulation of muscle contraction by troponin and calcium. Experiments are outlined to study the surface of native troponin from rabbit skeletal muscle, isolated and in combination with actin and tropomyosin, using a competitive labeling procedure. The relative reactivity of the epsilon-amino groups of lysine with acetic anhydride will be related to the amino acid sequence of each troponin component. Determination of which residues are "inaccessible" and which are freely "accessible" should elucidate the areas of each protein involved in conformational changes and interaction with the other proteins in the thin filament. Another area of research is the investigation of the interaction between actin and pancreatic DNase I. DNase I depolymerizes filamentous actin to form a 1:1 complex with monomeric actin and in doing so, the enzyme activity is inhibited. The effect of DNase I on the exchangeability of the actin bound nucleotide has been studied. Since actin-DNase I complex will not polymerize, the exchangeability of the actin nucleotide can be studied in physiological conditions that would normally result in polymerization of actin. This is of interest since much cellular actin is thought to be in a non-filamentous state.